For a semiconductor layer in a display device such as a liquid crystal display device, an electroluminescence display device or the like, amorphous silicon or low-temperature polysilicon is widely used. Along with realization of a larger size of display screen, display of higher definition, lower power consumption and the like, various oxide semiconductor materials have been developed.
An example of oxide semiconductor materials is indium-gallium-zinc-oxide (IGZO), which has characteristics of a high electron mobility, a small leak current and the like. Oxide semiconductor materials of various compositions that have been studied as having characteristics superior to those of IGZO include indium-gallium-oxide (IGO), gallium-zinc-oxide (GZO), zinc-tin-oxide (ZTO), indium-zinc-tin-oxide (IZTO), indium-gallium-zinc-tin-oxide (IGZTO) and the like.
In general, an oxide semiconductor material is applied onto a substrate formed of glass or the like by use of a film formation process of sputtering or the like and thus is formed into a thin film. Next, the resultant thin film is etched by use of a resist or the like as a mask, so that an electrode pattern is formed. The etching step may be performed by a wet method or a dry method. The wet method uses a liquid etching composition.
Among oxide semiconductor materials, an oxide containing indium, zinc, tin and oxygen has a superb chemical resistance, and thus is stable even when being exposed to various chemicals or various types of gas in steps of forming other films of other materials or in an etching step. However, such an oxide containing indium, zinc, tin and oxygen has a problem of being difficult to be processed by wet etching or the like.
A liquid etching composition that is used to form a pattern of an oxide semiconductor material by wet etching needs to have the following properties (1) through (5):
(1) Has a preferable etching rate (E.R.);
(2) Does not have the etching rate changed much when dissolving the oxide; namely, is stably usable for a long time and has a long life as a liquid chemical;
(3) Generates no deposit when dissolving the oxide;
(4) Does not leave much etching residue; and
(5) Does not corrode materials of wirings or other peripheral members.
The etching rate on an oxide semiconductor material is preferably 10 nm/min. or higher, more preferably 20 nm/min. or higher, and still more preferably 30 nm/min. or higher. The etching rate on an oxide semiconductor material is preferably 10000 nm/min. or lower, more preferably 5000 nm/min. or lower, and still more preferably 2000 nm/min. or lower. Among these ranges, 10 to 10000 nm/min. is preferable, 20 to 5000 nm/min. is more preferable, and 30 to 2000 nm/min. is still more preferable. When the etching rate is 10 to 10000 nm/min., the production efficiency is kept high and the etching operation is performed stably.
It is also preferable that when an oxide semiconductor material is dissolved in a liquid etching composition as the wet etching progresses, the etching rate is not changed much. This is very important for efficient industrial production in the case where a liquid etching composition is used for etching an oxide semiconductor material.
When deposit is generated in a liquid etching composition having an oxide semiconductor material dissolved therein, there is a possibility that the deposit is left as residue on the post-etching substrate. The residue may generate voids, cause insufficient adhesiveness, cause electricity leakage or wire breakage in various film formation steps after the dissolution. Any of these may result in insufficient characteristics of the display device. It is preferable that there is not much residue on the post-etching substrate.
When deposit is generated in a liquid etching composition having an oxide semiconductor material dissolved therein, the deposit may clog a filter provided for circulating the liquid etching composition. Filter exchange is a complicated process and may raise the cost. This requires the liquid etching composition to be disposed of before the deposit is generated even if the liquid etching composition is still usable. This shortens the period in which each liquid etching composition is usable and thus raises the cost of liquid etching compositions. In addition, the cost of processing the liquid waste is also raised.
For example, it is assumed that a liquid etching composition containing general oxalic acid (not containing component A described below) is used to etch an oxide semiconductor material containing zinc. In this case, there is a serious problem that zinc oxalate is deposited as a solid substance. In the case of a liquid etching composition containing general oxalic acid, the deposit is generated when zinc dissolved in the liquid etching composition has a concentration of about 10 ppm by mass.
Therefore, the liquid etching composition is required to generate no deposit even when an oxide containing zinc is dissolved therein. Specifically, zinc dissolved in the liquid etching composition has a concentration of preferably 10 ppm by mass or higher. Zinc dissolved in the liquid etching composition has a concentration of more preferably 100 ppm by mass or higher, and especially preferably 1000 ppm by mass or higher.
There is no specific upper limitation on the concentration of zinc. However, for a stable etching operation, zinc has a concentration of preferably 5000 ppm by mass or lower, more preferably 4000 ppm by mass or lower, and especially preferably 3000 ppm by mass or lower.
In general, wiring materials used in a display device such as a liquid crystal display device or the like are formed of copper (Cu), aluminum (Al), molybdenum (Mo), titanium (Ti) or the like. There is a possibility that a liquid etching composition contacts such wiring materials during the etching performed on an oxide semiconductor material. Therefore, it is preferable that the liquid etching composition has a low level of corrosiveness on the wiring materials. The liquid etching composition has a specific etching rate on the wiring materials of preferably 3 nm/min. or lower, more preferably 2 nm/min. or lower, and especially preferably 1 nm/min. or lower.
Patent Document 1 (Japanese Laid-Open Patent Publication No. 2001-155549) describes that indium-zinc-tin-oxide (IZTO) is etchable with hydrochloric acid.
Patent Document 2 (Japanese Laid-Open Patent Publication No. 2007-63649) describes that indium-zinc-tin-oxide (IZTO) is etchable with an aqueous solution of oxalic acid.
Patent Document 3 (Japanese Laid-Open Patent Publication No. 2006-77241) discloses a liquid etching composition usable for indium-zinc-oxide (IZO) or indium-tin-oxide (ITO). This liquid etching composition contains sulfuric acid as an oxidizer, a compound selected from phosphoric acid, nitric acid, acetic acid, perchloric acid, hydrogen peroxide, persulfate and the like as an assisting oxidizer, ammonium salt as an etching suppressor, and water.
Patent Document 4 (Japanese Laid-Open Patent Publication No. 2009-218513) discloses a liquid etching composition usable for indium-tin-oxide (ITO). This liquid etching composition contains sulfuric acid, a surfactant, and water.
Patent Document 5 (WO2008/32728) discloses a liquid etching composition usable for indium-tin-oxide (ITO). This liquid etching composition contains (a) oxalic acid, (b) naphthalenesulfonic acid condensate or a salt thereof, (c) at least one of hydrochloric acid, sulfuric acid, water-soluble amine, and a salt of any of these, and (d) water.